TL494

OK, we've all been bitching - silently or vocally - about the low 
SNR at SED. Here's a real, albeit small, design question.

The venerable PWM controller TL494 was widely used in older 
computer power supplies. It differs from the similar SG3524 in 
that, in addition to two error amplifiers, it has a dedicated 
dead time control (DTC) pin. It allows minimum dead time at 0V 
and zero duty cycle at around +3V.

In the PSU designs I've analyzed, the DTC pin is held at 0V in 
normal operation and is pulled high under fault conditions such 
as overvoltage or failed 'power good' signal.

I'm considering using the TL494 in a minor project with the roles 
of the DTC and error amps interchanged. The IC will provide free 
running push-pull pulse outputs to drive a power stage with a 
fixed duty cycle set by the DTC pin. There will be no feedback 
from the output Under normal conditions and the error amps will 
intervene only under fault conditions.

There's just one thing I'm not perfectly clear about from looking 
at the datasheet: Can the error amps override the fixed duty 
cycle setting in case of a fault?

Both inputs (DTC and COMP) act to reduce the output pulse width.

COMP is equivalent to DTC, except it has 0.7V offset to CT (instead of 
0.1V), and it has two error amps wired-OR on it.  And a slight current sink.

Best, semi-traditional way to use it, is: half bridge forward converter (add 
gate drivers and gate drive transformer; controller is secondary side, and 
needs an aux supply to start up), current shunt senses output filter 
inductor current (or rectifier/secondary ground return), error amp closes 
loop on that.  Err amp 2 is left disabled (strap inputs to REF/GND).  Add 
external op-amp for voltage error amp.  This breaks the infamous 
compensation problem that plagues voltage mode controls, and provides 
implicit current limiting (average current mode control).

Otherwise if you just need some PWM, it's a fine starting choice.  Easily 
frequency modulated, too (a somewhat less convenient basis for a resonant 
SMPS, perhaps?).  I've used it for induction heaters before.

Tim

-- 
Seven Transistor Labs, LLC
Electrical Engineering Consultation and Contract Design
Website: http://seventransistorlabs.com

"Pimpom" <Pimpom@invalid.com> wrote in message 
news:op30k3$pep$1@news.albasani.net...
> OK, we've all been bitching - silently or vocally - about the low SNR at 
> SED. Here's a real, albeit small, design question.
>
> The venerable PWM controller TL494 was widely used in older computer power 
> supplies. It differs from the similar SG3524 in that, in addition to two 
> error amplifiers, it has a dedicated dead time control (DTC) pin. It 
> allows minimum dead time at 0V and zero duty cycle at around +3V.
>
> In the PSU designs I've analyzed, the DTC pin is held at 0V in normal 
> operation and is pulled high under fault conditions such as overvoltage or 
> failed 'power good' signal.
>
> I'm considering using the TL494 in a minor project with the roles of the 
> DTC and error amps interchanged. The IC will provide free running 
> push-pull pulse outputs to drive a power stage with a fixed duty cycle set 
> by the DTC pin. There will be no feedback from the output Under normal 
> conditions and the error amps will intervene only under fault conditions.
>
> There's just one thing I'm not perfectly clear about from looking at the 
> datasheet: Can the error amps override the fixed duty cycle setting in 
> case of a fault? 

On Sun, 10 Sep 2017 14:58:32 +0530, Pimpom <Pimpom@invalid.com> wrote:

>OK, we've all been bitching - silently or vocally - about the low 
>SNR at SED. Here's a real, albeit small, design question.
>
>The venerable PWM controller TL494 was widely used in older 
>computer power supplies. It differs from the similar SG3524 in 
>that, in addition to two error amplifiers, it has a dedicated 
>dead time control (DTC) pin. It allows minimum dead time at 0V 
>and zero duty cycle at around +3V.
>
>In the PSU designs I've analyzed, the DTC pin is held at 0V in 
>normal operation and is pulled high under fault conditions such 
>as overvoltage or failed 'power good' signal.
>
>I'm considering using the TL494 in a minor project with the roles 
>of the DTC and error amps interchanged. The IC will provide free 
>running push-pull pulse outputs to drive a power stage with a 
>fixed duty cycle set by the DTC pin. There will be no feedback 
>from the output Under normal conditions and the error amps will 
>intervene only under fault conditions.
>
>There's just one thing I'm not perfectly clear about from looking 
>at the datasheet: Can the error amps override the fixed duty 
>cycle setting in case of a fault?

If you look at the data sheet, you'll see that the DTC and error 
amplifier outputs are orred, to turn off the outputs.

What is not normal about the TL494 is that these controls actually 
set the threshold of the cycle turn-on; At the start of the clock
period, switches are off until the oscillator output rises
sufficiently to reach the threshold set by both the DT and the outputs
of error amplifier control functions. 

The switches turn off at the end of the clock period, when the voltage
on the CT pin external capacitor reaches a maximum control threshold,
set by the internal oscillator.

A turn-on event can be over-ridden by the error amplifiers' internally
diode-orred output (visible at FB pin) during the clock cycle, turning
switches off prematurely, before the end of the clock cycle. Be aware,
however, that the turn-off is not latched in the internal logic. A
dominant error amp output or a controlling dead-time threshold, if
removed, can again allow the switches to turn on before the end of the
clock cycle.

Similar direct control of the output switches can be effected through
the Vz pin.

This lack of cyclic latching can complicate any externally-developed
protection function that is expected to operate on a cycle-by-cycle
basis. In order to do so, this protection signal must be maintained
externally, for the full clock cycle. 

If the protection signal is continually maintained, there is no issue,
save the lack of synchronism or slow-restart precautions, on release.

RL

On 9/10/2017 7:56 PM, Tim Williams wrote:
> Both inputs (DTC and COMP) act to reduce the output pulse width.
> 
> COMP is equivalent to DTC, except it has 0.7V offset to CT (instead of
> 0.1V), and it has two error amps wired-OR on it.  And a slight current sink.
> 
> Best, semi-traditional way to use it, is: half bridge forward converter (add
> gate drivers and gate drive transformer; controller is secondary side, and
> needs an aux supply to start up), current shunt senses output filter
> inductor current (or rectifier/secondary ground return), error amp closes
> loop on that.  Err amp 2 is left disabled (strap inputs to REF/GND).  Add
> external op-amp for voltage error amp.  This breaks the infamous
> compensation problem that plagues voltage mode controls, and provides
> implicit current limiting (average current mode control).
> 
> Otherwise if you just need some PWM, it's a fine starting choice.  Easily
> frequency modulated, too (a somewhat less convenient basis for a resonant
> SMPS, perhaps?).  I've used it for induction heaters before.
> 
> Tim
> 
I had more than one application in mind, none of which is fully 
thought out yet. One of them is a small simple inverter at mains 
frequency instead of the tens of kHz these devices usually 
operate at.

In this application, the IC will serve as an oscillator with a 
push-pull output at a preset duty cycle. The output will go to a 
pair of transistors which in turn drives a step-up transformer. 
There will be no attempt to regulate the final output voltage by 
PWM except to limit the max current drawn by the transistors.

There are many designs for a basic inverter on the internet but 
most of them operate at near 50-50 duty cycle and I don't want 
that. This is where the TL494 comes in. It's cheaper than an MCU, 
can be 'programmed with a couple of resistors and can run 
straight off a 12V battery.

On Sunday, 10 September 2017 20:17:03 UTC+1, Pimpom  wrote:
> On 9/10/2017 7:56 PM, Tim Williams wrote:
> > Both inputs (DTC and COMP) act to reduce the output pulse width.
> > 
> > COMP is equivalent to DTC, except it has 0.7V offset to CT (instead of
> > 0.1V), and it has two error amps wired-OR on it.  And a slight current sink.
> > 
> > Best, semi-traditional way to use it, is: half bridge forward converter (add
> > gate drivers and gate drive transformer; controller is secondary side, and
> > needs an aux supply to start up), current shunt senses output filter
> > inductor current (or rectifier/secondary ground return), error amp closes
> > loop on that.  Err amp 2 is left disabled (strap inputs to REF/GND).  Add
> > external op-amp for voltage error amp.  This breaks the infamous
> > compensation problem that plagues voltage mode controls, and provides
> > implicit current limiting (average current mode control).
> > 
> > Otherwise if you just need some PWM, it's a fine starting choice.  Easily
> > frequency modulated, too (a somewhat less convenient basis for a resonant
> > SMPS, perhaps?).  I've used it for induction heaters before.
> > 
> > Tim
> > 
> I had more than one application in mind, none of which is fully 
> thought out yet. One of them is a small simple inverter at mains 
> frequency instead of the tens of kHz these devices usually 
> operate at.
> 
> In this application, the IC will serve as an oscillator with a 
> push-pull output at a preset duty cycle. The output will go to a 
> pair of transistors which in turn drives a step-up transformer. 
> There will be no attempt to regulate the final output voltage by 
> PWM except to limit the max current drawn by the transistors.
> 
> There are many designs for a basic inverter on the internet but 
> most of them operate at near 50-50 duty cycle and I don't want 
> that. This is where the TL494 comes in. It's cheaper than an MCU, 
> can be 'programmed with a couple of resistors and can run 
> straight off a 12V battery.

Such an invertor would suffer deregulation due to battery voltage & transformer copper drops, which added together makes a fair amount of swing. If your app can tolerate that why not go simpler & use an oscillating relay.


NT

On 9/10/2017 11:02 PM, legg wrote:
> On Sun, 10 Sep 2017 14:58:32 +0530, Pimpom <Pimpom@invalid.com> wrote:
> 
>> OK, we've all been bitching - silently or vocally - about the low
>> SNR at SED. Here's a real, albeit small, design question.
>>
>> The venerable PWM controller TL494 was widely used in older
>> computer power supplies. It differs from the similar SG3524 in
>> that, in addition to two error amplifiers, it has a dedicated
>> dead time control (DTC) pin. It allows minimum dead time at 0V
>> and zero duty cycle at around +3V.
>>
>> In the PSU designs I've analyzed, the DTC pin is held at 0V in
>> normal operation and is pulled high under fault conditions such
>> as overvoltage or failed 'power good' signal.
>>
>> I'm considering using the TL494 in a minor project with the roles
>> of the DTC and error amps interchanged. The IC will provide free
>> running push-pull pulse outputs to drive a power stage with a
>> fixed duty cycle set by the DTC pin. There will be no feedback
>>from the output Under normal conditions and the error amps will 
>> intervene only under fault conditions.
>>
>> There's just one thing I'm not perfectly clear about from looking
>> at the datasheet: Can the error amps override the fixed duty
>> cycle setting in case of a fault?
> 
> If you look at the data sheet, you'll see that the DTC and error
> amplifier outputs are orred, to turn off the outputs.
> 
> What is not normal about the TL494 is that these controls actually
> set the threshold of the cycle turn-on; At the start of the clock
> period, switches are off until the oscillator output rises
> sufficiently to reach the threshold set by both the DT and the outputs
> of error amplifier control functions.
> 
> The switches turn off at the end of the clock period, when the voltage
> on the CT pin external capacitor reaches a maximum control threshold,
> set by the internal oscillator.
> 
> A turn-on event can be over-ridden by the error amplifiers' internally
> diode-orred output (visible at FB pin) during the clock cycle, turning
> switches off prematurely, before the end of the clock cycle. Be aware,
> however, that the turn-off is not latched in the internal logic. A
> dominant error amp output or a controlling dead-time threshold, if
> removed, can again allow the switches to turn on before the end of the
> clock cycle.
> 
> Similar direct control of the output switches can be effected through
> the Vz pin.
> 
> This lack of cyclic latching can complicate any externally-developed
> protection function that is expected to operate on a cycle-by-cycle
> basis. In order to do so, this protection signal must be maintained
> externally, for the full clock cycle.
> 
> If the protection signal is continually maintained, there is no issue,
> save the lack of synchronism or slow-restart precautions, on release.
> 
> RL
> 
Thanks. I can see those things in the internal block diagram now 
that you've pointed them out.

To reiterate, there will be no attempt to regulate the output 
voltage by PWM under normal load. I thought I'd feed a signal to 
the error amp proportional to the current drawn by the power 
stage and that will have an effect only if the current crosses a 
preset threshold..

On 9/11/2017 12:52 AM, tabbypurr@gmail.com wrote:
> On Sunday, 10 September 2017 20:17:03 UTC+1, Pimpom  wrote:
>> On 9/10/2017 7:56 PM, Tim Williams wrote:
>>> Both inputs (DTC and COMP) act to reduce the output pulse width.
>>>
>>> COMP is equivalent to DTC, except it has 0.7V offset to CT (instead of
>>> 0.1V), and it has two error amps wired-OR on it.  And a slight current sink.
>>>
>>> Best, semi-traditional way to use it, is: half bridge forward converter (add
>>> gate drivers and gate drive transformer; controller is secondary side, and
>>> needs an aux supply to start up), current shunt senses output filter
>>> inductor current (or rectifier/secondary ground return), error amp closes
>>> loop on that.  Err amp 2 is left disabled (strap inputs to REF/GND).  Add
>>> external op-amp for voltage error amp.  This breaks the infamous
>>> compensation problem that plagues voltage mode controls, and provides
>>> implicit current limiting (average current mode control).
>>>
>>> Otherwise if you just need some PWM, it's a fine starting choice.  Easily
>>> frequency modulated, too (a somewhat less convenient basis for a resonant
>>> SMPS, perhaps?).  I've used it for induction heaters before.
>>>
>>> Tim
>>>
>> I had more than one application in mind, none of which is fully
>> thought out yet. One of them is a small simple inverter at mains
>> frequency instead of the tens of kHz these devices usually
>> operate at.
>>
>> In this application, the IC will serve as an oscillator with a
>> push-pull output at a preset duty cycle. The output will go to a
>> pair of transistors which in turn drives a step-up transformer.
>> There will be no attempt to regulate the final output voltage by
>> PWM except to limit the max current drawn by the transistors.
>>
>> There are many designs for a basic inverter on the internet but
>> most of them operate at near 50-50 duty cycle and I don't want
>> that. This is where the TL494 comes in. It's cheaper than an MCU,
>> can be 'programmed with a couple of resistors and can run
>> straight off a 12V battery.
> 
> Such an invertor would suffer deregulation due to battery voltage & transformer copper drops, which added together makes a fair amount of swing. If your app can tolerate that

Yes they can. Those effects are fully anticipated.

> why not go simpler & use an oscillating relay.
> 
You mean like those electromechanical vibrators from the vacuum 
tube days? That's a bit too crude for my purpose. I want a fairly 
stable frequency, presetable duty cycle and no mechanical contact 
with its attendant problems.

On Sunday, 10 September 2017 20:45:04 UTC+1, Pimpom  wrote:
> On 9/11/2017 12:52 AM, tabbypurr wrote:
> > On Sunday, 10 September 2017 20:17:03 UTC+1, Pimpom  wrote:
> >> On 9/10/2017 7:56 PM, Tim Williams wrote:

> >>> Both inputs (DTC and COMP) act to reduce the output pulse width.
> >>>
> >>> COMP is equivalent to DTC, except it has 0.7V offset to CT (instead of
> >>> 0.1V), and it has two error amps wired-OR on it.  And a slight current sink.
> >>>
> >>> Best, semi-traditional way to use it, is: half bridge forward converter (add
> >>> gate drivers and gate drive transformer; controller is secondary side, and
> >>> needs an aux supply to start up), current shunt senses output filter
> >>> inductor current (or rectifier/secondary ground return), error amp closes
> >>> loop on that.  Err amp 2 is left disabled (strap inputs to REF/GND).  Add
> >>> external op-amp for voltage error amp.  This breaks the infamous
> >>> compensation problem that plagues voltage mode controls, and provides
> >>> implicit current limiting (average current mode control).
> >>>
> >>> Otherwise if you just need some PWM, it's a fine starting choice.  Easily
> >>> frequency modulated, too (a somewhat less convenient basis for a resonant
> >>> SMPS, perhaps?).  I've used it for induction heaters before.
> >>>
> >>> Tim
> >>>
> >> I had more than one application in mind, none of which is fully
> >> thought out yet. One of them is a small simple inverter at mains
> >> frequency instead of the tens of kHz these devices usually
> >> operate at.
> >>
> >> In this application, the IC will serve as an oscillator with a
> >> push-pull output at a preset duty cycle. The output will go to a
> >> pair of transistors which in turn drives a step-up transformer.
> >> There will be no attempt to regulate the final output voltage by
> >> PWM except to limit the max current drawn by the transistors.
> >>
> >> There are many designs for a basic inverter on the internet but
> >> most of them operate at near 50-50 duty cycle and I don't want
> >> that. This is where the TL494 comes in. It's cheaper than an MCU,
> >> can be 'programmed with a couple of resistors and can run
> >> straight off a 12V battery.
> > 
> > Such an invertor would suffer deregulation due to battery voltage & transformer copper drops, which added together makes a fair amount of swing. If your app can tolerate that
> 
> Yes they can. Those effects are fully anticipated.
> 
> > why not go simpler & use an oscillating relay.
> > 
> You mean like those electromechanical vibrators from the vacuum 
> tube days? That's a bit too crude for my purpose. I want a fairly 
> stable frequency, presetable duty cycle and no mechanical contact 
> with its attendant problems.

Yes, those. FWIW duty cycle was adjustable by changing the spacing, at least on some units.


NT

On Mon, 11 Sep 2017 00:46:54 +0530, Pimpom <Pimpom@invalid.com> wrote:

>On 9/10/2017 7:56 PM, Tim Williams wrote:
>> Both inputs (DTC and COMP) act to reduce the output pulse width.
>> 
>> COMP is equivalent to DTC, except it has 0.7V offset to CT (instead of
>> 0.1V), and it has two error amps wired-OR on it.  And a slight current sink.
>> 
>> Best, semi-traditional way to use it, is: half bridge forward converter (add
>> gate drivers and gate drive transformer; controller is secondary side, and
>> needs an aux supply to start up), current shunt senses output filter
>> inductor current (or rectifier/secondary ground return), error amp closes
>> loop on that.  Err amp 2 is left disabled (strap inputs to REF/GND).  Add
>> external op-amp for voltage error amp.  This breaks the infamous
>> compensation problem that plagues voltage mode controls, and provides
>> implicit current limiting (average current mode control).
>> 
>> Otherwise if you just need some PWM, it's a fine starting choice.  Easily
>> frequency modulated, too (a somewhat less convenient basis for a resonant
>> SMPS, perhaps?).  I've used it for induction heaters before.
>> 
>> Tim
>> 
>I had more than one application in mind, none of which is fully 
>thought out yet. One of them is a small simple inverter at mains 
>frequency instead of the tens of kHz these devices usually 
>operate at.
>
>In this application, the IC will serve as an oscillator with a 
>push-pull output at a preset duty cycle. The output will go to a 
>pair of transistors which in turn drives a step-up transformer. 
>There will be no attempt to regulate the final output voltage by 
>PWM except to limit the max current drawn by the transistors.
>
>There are many designs for a basic inverter on the internet but 
>most of them operate at near 50-50 duty cycle and I don't want 
>that. This is where the TL494 comes in. It's cheaper than an MCU, 
>can be 'programmed with a couple of resistors and can run 
>straight off a 12V battery.

I've done the exact same thing you are planning to do, it was for a
isolated 485 bus implemtation. Fixed frequency push pull.  Works well.
One observation in the differences of the 494 and 3525 is that 3525
has a shutdown pin , soft start, totempol outputs, and sync pins.
But for the price, it's an easy push pull oscillator.

Cheers

"Martin Riddle" <martin_ridd@verizon.net> wrote in message 
news:qiibrcp4ip6bqkdjmepn4qrlo45un1vvid@4ax.com...
> I've done the exact same thing you are planning to do, it was for a
> isolated 485 bus implemtation. Fixed frequency push pull.  Works well.
> One observation in the differences of the 494 and 3525 is that 3525
> has a shutdown pin , soft start, totempol outputs, and sync pins.
> But for the price, it's an easy push pull oscillator.

If you're looking for the TI equivalent: TL598.  Or Unitrode/TI UC3525 (or 
UCC, I forget).

Fine for direct driving transformers, too, but add schottky clamp diodes (to 
+V/GND) to handle reactive current -- they're bipolar, not like today's CMOS 
gate drivers.

Tim

-- 
Seven Transistor Labs, LLC
Electrical Engineering Consultation and Contract Design
Website: http://seventransistorlabs.com